The present invention generally involves oil well drilling bits, and more particularly discloses the large diameter drilling bits commonly termed, "top hole" bits used to drill the very first section of borehole beginning at the ground surface, and are also used for drilling casing openings. The top hole bits generally range in size from about 18 inches up to about 28 inches and are very cumbersome and heavy. Generally, top hole bits comprise either four-cutter bits commonly called, "cross cutter" bits, or else they comprise tri-cone rolling cutter bits such as that disclosed herein. The general method of manufacturing large tri-cone rolling cutter bits is in the segmented arc construction method. This method utilizes three 120-degree arcuate lug sections, each comprising one-third of the drill bit body and an individual leg portion with a bearing journal thereon. These three arcuate sections are usually forged and then machined to form the bearing surfaces on the bearing journals and the mating surfaces along each edge where the three sections are welded together. Prior to joining the three arcuate sections to form a cylindrical body, the cutter assembly with bearings and retention means must first be mounted on the inwardly projecting bearing journals because of the impossibility of so mounting the cutters after the three arcuate sections have been welded together.
After the bearing assemblies and cutters are mounted on the machined bearing journals, the three lugs are then placed in a welding jig and welded to each other to form the cylindrical bit body. After this welding has occurred a tapered thread is machined on the upper end of the bit, which tapered threaded end is commonly referred to as the "pin" end, and the bit is ready for use. The difficulties with this assembly method in manufacturing tophole or large diameter bits is that because of the size of these bits, minor variations and tolerances in alignments of the three lug sections results in substantial final errors in the bit specifications and dimensions. The forged lug sections of the prior art bits are relatively rough and inaccurate and the machining of the mating surfaces likewise is difficult to control to close tolerances. When the three lug sections are eventually welded together to form a single cylindrical structure with cutter assemblies already permanently mounted thereon, most often the results are that the cutters are not only non-concentric about the rotational axis of the bit, but the cutters also extend different distances downward from the pin end of the bit. Thus, one cutter may extend further downward than the other two and may provide almost all of the cutting action on the borehole bottom until that cutter is worn substantially to match the shorter two cutters. This tends to cause premature failure of either the bearings or the cutter shell on that one cutting assembly, which in turn will result in early failure of the bit. Likewise, radial placement of the lugs is difficult to control and may result in the drill bit cutting under gage, which is undesirable because of the borehole being smaller than necessary and smaller than specified. Also, if one cutter extends radially further outward than either one or both of the other cutters, a resulting effect could be "tracking" of the cutting teeth on the cutters and eventual gyration or orbiting action of the bit about the rotational axis thereby substantially eliminating the cutting efficiency of the bit.
The present invention eliminates these disadvantages found in the big diameter or top hole bits by providing a structure that is much more easily assembled and in which tolerances can be closely controlled during assembly of the bit. Further disclosure is made of a method of assembling the bit to provide extremely close tolerances in the radial and axial directions of the cutter assemblies. Also disclosed is a method of varying the offset of the cutter axes by relatively simple and inexpensive means during construction of the bit. The invention is achieved by utilizing a cast bit body which is formed in a single section and which has three platform areas formed thereon for receiving three independent cutter assemblies which are then welded to the cast iron body. Each cutter assembly has a countersunk alignment point which is engaged by the hydraulic or mechanical alignment tool which forms a part of this invention. The alignment tool fixes the cutter assemblies in place on the cast metal body whereupon they are welded in place to form a highly accurate, close tolerance, large diameter tri-cone drilling bit.